1. Field of the Invention
This invention relates generally to roofing structures and, more particularly but not exclusively, to roofing elements which co-operate with each other to provide resistance to movement, especially under windy conditions.
2. Description of the Prior Art
Roof constructions wherein a waterproof membrane is laid under or over a layer of insulation, both layers being held in place by a ballast system are well known, especially in flat roof constructions. The membrane is commonly made from rubber, plastics or some other type of waterproof material. Various techniques may be used for installing the membrane, most commonly, it is loose laid, either under or over the roof insulation. These materials are then held in place by ballast.
Frequently, flat roof structures are recessed relative to the top edge of the wall member surrounding the roof thus forming a parapet. When wind passes over the parapet, a vacuum effect along the surface of the roof is generated, the effect being particularly pronounced along the perimeter area adjacent to the parapet. This vacuum effect causes substantial uplift forces on the roof covering, and can result in uplift of the ballast material, even heavy concrete blocks.
The ballast serves a number of purposes; it acts to protect the membrane from puncturing or tearing; it helps shield the membrane from the deleterious effects of the sun's ultraviolet rays; it can prevent flame spread and damage from hot embers; but more particularly, it protects the membrane from the uplift forces which develop from naturally occurring winds.
A number of ballast systems have been used in the past, the simplest being a layer of loose laid, smooth stones. Other systems have included a layer of paving slabs or ballast blocks, or a layer of tongue and groove `boards` made from concrete.
In general, although convenient, the use of loose stones as ballast has not proved satisfactory. The stones are liable to move about and in doing so scour the membrane surface and leave areas of the membrane exposed thereby reducing the lifespan of the membrane.
Conventional ballast blocks also have their drawbacks. As discussed above, the blocks may be subject to uplift from wind forces as well as from the freezing of any water which may lie on the roof surface. This uplift can result in rotation of the blocks, again scouring the membrane surface and exposing the membrane to the action of the weather.
Efforts to overcome the disadvantages associated with conventional ballast blocks have led to several proposals in which the ballast blocks co-operate with each other in an attempt to eliminate or reduce relative movement therebetween.
For example, U.S. Pat. No. 4,506,483 to Phalen (assignee: Roofblok Limited) discloses a roof construction comprising ballast blocks, each block having two parallel edges bevelled at substantially identical angles from the vertical, the remaining two edges being substantially vertical. In use, the blocks are laid in a specified pattern adjacent each rectangular corner of the roof so that downwardly and outwardly bevelled edges are adjacent each roof edge at the corner. Means for clipping or clamping the block edges to the deck are required.
U.S. Pat. No. 4,535,579 to Burgoyne et al (assignee: Roofblok Limited) discloses a roof construction comprising ballast blocks having the same shape as those disclosed in U.S. Pat. No. 4,506,483 referred to above, the blocks being characterised by their compressive strength, flexural strength, density and freeze-thaw cycle properties.
U.S. Pat. No. 4,899,514 to Brookhart discloses ballast blocks in the form of planar plate members, each plate member having a top and bottom surface, front and rear end portions and oppositely disposed lateral edges. The end portions co-operate in overlapping relationship with the corresponding end portions of like ballast blocks.
While the roofing elements of the prior art discussed above help to reduce the relative movement between the elements often caused by the wind or by persons needing access to the roof, for example to undertake repairs or install TV aerials, etc., the problem is not entirely eliminated. For example, it is still possible for adjacent rows of roofing elements to slide or be uplifted relative to each other. It is against this background that the present invention was devised.